In general, position encoders are devices which determine the instantaneous physical position of a movable object with respect to a fixed reference point, and translate such position information into a form that can be utilized by a processing tool. A position encoder typically transforms position information into an electrical signal, and provides the electrical signal to an analog or digital signal processor. Position encoders may determine angular position, as in the case of a rotatable shaft or toroidal structure, or they may determine linear position, as in the case of a slidable control actuator. An ideal position encoder produces an output signal that is a linear function of the position of the movable object. Instantaneous position information, sampled over time, may be used to determine higher derivatives of position such as velocity and acceleration.
Typical position encoders operate either mechanically, electrically (e.g., capacitive sensors), optically or magnetically. A mechanical encoder relies upon physical contact with the movable object; actuators on the movable object physically interact with an electromechanical transducer to produce an electrical signal. An optical encoder receives light reflected from illuminated markings associated with the movable object and translates variations in the received light into an electrical signal. A magnetic encoder typically utilizes either fluxgate sensors, magneto-resistive sensors, giant magneto-resistive sensors, or Hall effect sensors. A fluxgate sensor magnetic encoder uses fluxgate sensors to detect the magnetic field generated by magnetic elements attached to the movable object, and translates aspects of the magnetic field such as magnitude and polarity into an electrical signal corresponding to the position and direction of motion of the object. A Hall effect sensor magnetic encoder translates the Hall effect of a magnetic field on a current carrying conductor to produce a signal corresponding to the position of the object. Fluxgate position encoders are several orders of magnitude more sensitive than Hall effect position encoders and are thus preferred in applications where it may be difficult to have the sensors in close proximity of the magnetic element producing the magnetic field. For example, in an application to determine the angular position of an automobile tire incorporating a magnetic element, the close proximity that a Hall effect sensor requires is difficult to maintain because of the harsh environment created by road dirt, oil, grease, ice and snow, whereas a fluxgate sensor can operate at a distance of several inches.
A fluxgate sensor includes one or more turns of an electrical conductor wound about a core, which is disposed along a sensing axis. The core may be any magnetically saturating material, including highly permeable materials such as alloys of iron and nickel. Saturable alloys exhibiting high permeability and low coercive strength are preferred. An external driving circuit alternately drives the sensor into saturation in one polarity and then into the opposite polarity. An improved fluxgate driving circuit is described and claimed in my copending application, U.S. application Ser. No. 09/314,322, filed contemporaneously herewith, and assigned to the present assignee (Attorney Docket No. ADL-092). The external driving circuit drives current through the windings in one direction until the core saturates. Once the core saturates, the driving circuit reverses current in the windings until the core saturates in the opposite polarity. In the absence of an external magnetic field, the amount of time the driving circuit drives current in each direction is the same; i.e., the current waveform through the windings as a function of time is symmetrical. The presence of an external magnetic field "helps" (i.e., enhances) the saturation of the core in one polarity, while the external magnetic field impedes the saturation of the core in the opposite polarity. Thus, in the presence of an external magnetic field, the waveform of the current through the windings as a function of time is asymmetrical, since saturation occurs more quickly for the polarity of the saturation enhanced by the external field. The amount of asymmetry can be used to determine characteristics of the external magnetic field, such as magnitude and direction.
Because of the bipolar nature of the magnetic element, a fluxgate sensor having a fixed position and orientation at the perimeter of a rotating magnetic element produces a periodic sinusoidal output. Similarly, a fluxgate sensor having a fixed position and orientation alongside a magnetic element, movable along its polar axis, produces approximately one cycle of a sinusoid as the magnetic element moves from pole to pole past the fluxgate sensor. Prior art fluxgate position encoders map the resulting sinusoid from the fluxgate sensor to an output signal that is a linear function of the position of the magnetic element. Such mapping requires a significant amount of processing resources and is subject to error, since the resulting sinusoid is not a true closed-form sinusoid and the resulting sinusoid tends to change shape due to various factors such as temperature and shaft run-out (i.e., movement of the axis of rotation). As the sinusoid changes shape, the output of the mapping function will be less than linear and will introduce an error in the position representation. The mapping function may be designed to be adaptive so as to compensate for such variations, but an adaptive mapping function further increases the mapping complexity.
Since a sinusoid is substantially linear in the region of its zero crossings (i.e., where the sinusoid changes from positive to negative and vice versa), some prior art fluxgate position encoders utilize the fluxgate sensor output directly, and limit the range of motion of the magnetic element, so as to constrain the sensor output to the linear range. The limited range of motion is a significant disadvantage to this type of encoder. Further, even within the limited range of motion, the error (with respect to a true linear transfer function) increases as the motion of the magnetic element moves the output away from the sinusoid zero crossing.
A further disadvantage to prior art position encoders is a sensitivity to run-out. Run-out is defined as any movement of the movable object from its intended path of movement. For example, in a rotational position encoder, the movable object is typically a shaft that rotates about a rotational axis, such that the outer perimeter of the shaft maintains a circular envelope as the shaft rotates. As shaft bearings wear out over time, the rotation about the ideal rotational axis may become eccentric, such that the outer perimeter of the shaft traces an irregular envelope as the shaft rotates. This deviation from the ideal circular envelope is known in the art as run-out. For a linear position encoder, the movable object ideally moves along a linear axis. Run-out occurs when the motion of the movable object deviates from this linear axis of movement (e.g., side to side movement).
It is an object of this invention to provide a position encoder that substantially overcomes or reduces the aforementioned disadvantages while providing other advantages which will be evident hereinafter.